Level shifter for a time-varying input

ABSTRACT

A level shifter circuit for coupling a first circuit, that uses a first supply voltage, with a second circuit, that uses a second supply voltage, includes an input node to receive an input signal and an output node to output to a level-shifted output signal corresponding with the input signal. An idle state on the input node corresponds with a particular binary logic value that is maintained for a first time period, and which is detected by a detection sub-circuit. Further, the level shifter circuit includes a first inverter that uses the second supply voltage, and has a feedback path between the input and output of the first inverter. The feedback path includes a first resistive element and a first transmission gate. The first transmission gate is configurable to open the feedback path when the detection sub-circuit detects an idle state on the input node of the level shifter circuit.

BACKGROUND

The present disclosure relates to level shifting circuits for shifting avoltage from one voltage level to another level and, more particularly,to level shifting circuits for shifting a time-varying input voltage.

In analog circuits, input and output voltages can be any value in arange. In contrast, the input and output voltages in digital logiccircuits are required to be one of two values corresponding with a logicone or a logic zero. For example, five volts may correspond with a logicone and zero volts may correspond with a logic zero. This example is asimplification because precise voltages are not readily attainable inpractical circuits and some tolerance is allowed. As an example, anyvoltage between 3.5 and 5.0 volts might correspond with a logic one andany voltage between 0.0 and 0.8 volts might correspond with a logiczero. The voltage ranges for logic levels can also be expressed in termsof a supply voltage. For example, a voltage that is 70 to 100 percent ofthe supply voltage might correspond with a logic one, while a voltagethat is zero to 16 percent of the supply voltage might correspond with alogic zero. Different logic families and different technologies usedifferent supply voltages. It is common in the art to refer to thevoltage levels for a particular logic family and technology using asingle value, i.e., the voltage level corresponding with the logic one.For example, a digital logic circuit may be described as 5V, 3.3V, or1.7V logic.

Known circuits use a variety of voltage levels. Circuits using newvoltages levels are introduced from time to time. A first digital logiccircuit that uses a first voltage level can have a need to communicatewith a second digital logic circuit that uses a second, differentvoltage level. Accordingly, a circuit at the interface between the firstand second digital logic circuits that shifts the voltage levels used byone circuit to the voltages levels used by the other circuit enables thetwo circuits to communicate.

Digital logic circuits may use metal oxide semiconductor field-effect(MOSFET or MOS) transistors. MOSFET transistors may be n-channel MOSFETS(NMOS) or p-channel MOSFETS (PMOS). In addition, digital logic circuitsmay use complementary metal oxide semiconductor (CMOS) technology. InCMOS technology, both NMOS and PMOS transistors are present.

SUMMARY

Various embodiments of the present disclosure are directed to a levelshifter circuit for coupling a first circuit with a second circuit,where the first circuit uses a first supply voltage and the secondcircuit uses a second supply voltage. According to various embodiments,the level shifter circuit includes an input node to receive an inputsignal and an output node to output to a level-shifted output signalcorresponding with the input signal. In addition, the level shiftercircuit includes a detection sub-circuit to detect an idle state on theinput node. An idle state is a state that corresponds with a particularbinary logic value that is maintained for a first time period. Further,the level shifter circuit includes a first inverter that uses the secondsupply voltage. The first inverter has an input and an output. The levelshifter circuit also includes a feedback path between the input andoutput of the first inverter. The feedback path includes a firstresistive element and a first transmission gate. The first transmissiongate is configurable to open the feedback path when the detectionsub-circuit detects an idle state on the input node of the level shiftercircuit.

In various embodiments, the detection sub-circuit can include acapacitive element and a second resistive element. The first time periodof the idle state is determined by the capacitive element and the secondresistive element. In various embodiments, the detection sub-circuitincludes a second resistive element having a first and a second node,and a capacitive element. The capacitive element is connected betweenthe first node and a negative rail of a supply voltage. In addition, thedetection sub-circuit can include a first switching device seriallyconnected between the second node and a positive rail of the supplyvoltage. For example, the first switching device can be a PFET pull-upstack. Further, the detection sub-circuit can include a second switchingdevice connected between the first node and the negative rail of thesupply voltage, and in parallel with the capacitive element. Forexample, the second switching device can be an NFET. When the firstswitching device is closed, the capacitive element, the second resistiveelement, and the first switching device are serially connected betweenpositive and negative rails of a supply voltage to provide a path tocharge the capacitive element. Moreover, when the second switchingdevice is closed, the second switching device is connected between thefirst node and the negative rail of the supply voltage, providing a pathto discharge the capacitive element. The first and second switchingdevices are operated by the input signal on the input node, such thatwhen the first switching device is open, the second switching device isclosed. Additionally, the voltage at the first node, which may bereferred to as a “shutoff node,” provides an indication as to whetherthe input node is in the idle state.

According to various embodiments of the level shifter circuit, the inputof the first inverter is coupled with a multiplexer, the multiplexerbeing configurable to select a first or a second input signal.Additionally, the first input signal is input to the input node of thelevel shifter circuit and the multiplexer is configured to select thesecond input signal when the detection sub-circuit detects an idle stateon the input node.

In various embodiments, the level shifter circuit can include a controlsignal generation sub-circuit to generate a control signal in responseto detection of the idle state on the input node.

An idle state is a state that corresponds with a particular binary logicvalue that is maintained for a first time period. The particular binarylogic value can either a logic one or a logic zero.

In various embodiments, the level shifter circuit includes a capacitiveelement coupled between the input node and the input of the firstinverter. The capacitive element is configured to pass an AC componentof the input signal and to block a DC component of the input signal.

Additional embodiments are directed to methods and design structures fora level shifter circuit for coupling a first circuit with a secondcircuit, where the first circuit uses a first supply voltage and thesecond circuit uses a second supply voltage.

The above summary is not intended to describe each illustratedembodiment or every implementation of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an AC level shifter circuit according to variousembodiments.

FIG. 2 depicts various waveforms for the circuit of FIG. 1 according tovarious embodiments.

FIGS. 3A and 3B depict an AC level shifter circuit according to variousembodiments.

FIG. 4 depicts various waveforms for the circuit of FIGS. 3A and 3Baccording to various embodiments.

FIG. 5 is a flow diagram of a design process used in semiconductordesign, manufacture, or test.

The same numbers may be used in the Figures and the Detailed Descriptionto refer to the same devices, parts, components, steps, operations, andthe like.

While the concepts and embodiments described in this disclosure areamenable to various modifications and alternative forms, specificsthereof have been shown by way of example in the drawings and will bedescribed in detail. It should be understood, however, that theintention is not to limit the claims to the particular embodimentsdescribed. On the contrary, the intention is to cover all modifications,equivalents, and alternatives falling within the spirit and scope of theinvention.

DETAILED DESCRIPTION

Aspects of this disclosure relate to level shifting circuits forshifting a voltage from one voltage level to another level and, moreparticularly, to level shifting circuits for shifting a time-varyinginput voltage. While the present disclosure is not necessarily limitedto such applications, various aspects of the disclosure may beappreciated through a discussion of various examples using this context.

FIG. 1 depicts an AC level shifter circuit 100 for level shifting atime-varying input signal according to various embodiments. An exampleof a time-varying input signal is an input signal that may be either ahigh voltage corresponding with a logic one or a low voltagecorresponding with a logic zero. The time periods between transitionsfrom a high-to-low voltage and low-to-high voltage of an input signalmay vary. A time-varying input signal may transition with every clockcycle or may transition only after a relatively long time period. Atime-varying input signal may transition only after a relatively longtime period because the input is a sequence of unchanging digitalvalues. In addition, a time-varying input signal may transition onlyafter a relatively long time period because the input is inactive oridle. When the time-varying input signal is idle for relatively longtime period, the AC level shifter circuit 100 can consume a large amountof power due to DC current paths.

The circuit 100 level shifts a time-varying input signal IN_VCC to atime-varying output signal OUT_VDD. In particular, the circuit 100 levelshifts a time-varying input signal IN_VCC that is output from a firstdigital logic circuit (not shown) that uses VCC as a supply voltage. Thecircuit 100 level shifts the input signal to the output signal OUT_VDDthat can be provided to a second digital logic circuit (not shown) thatuses VDD as a supply voltage. In the example of FIG. 1, a first voltagelevel of the first circuit is less than a second voltage level of thesecond circuit, e.g. VCC=0.8V<VDD=1.1V. Accordingly, one example of thecircuit 100 up-shifts the input signal. In other examples, the circuit100 is capable of down-shifting the input signal.

As shown in FIG. 1, the AC level shifter circuit 100 includes severalreference nodes that are helpful for describing the structure andfunction of the circuit. Scanning the figure from left to right, thesenodes include: input node N_IN, node N1 _(—) vdd, node N2 _(—) vdd, nodeN3 _(—) vdd, and output node N_OUT.

Still referring to FIG. 1 and again scanning the figure from left toright, the circuit 100 includes a first transmission gate constructedfrom parallelly arranged PMOS transistor T0 and NMOS transistor T1. Thefirst transmission gate T0/T1 has a first terminal connected to inputnode N_IN and a second terminal connected to node N1 _(—) vdd. The inputsignal IN_VCC is input to the first transmission gate. The firsttransmission gate T0/T1 is enabled when EN_VDD is high and itscomplement en_b_vdd is low.

The circuit 100 includes an NMOS transistor T2, having both gateterminal and a source/drain terminal coupled to VSS, and capacitors C1and C2. The capacitors C1 and C2 are DC blocking or AC couplingcapacitors. The transistor T2 provides additional DC blockingcapacitance. In an embodiment, the capacitors C1 and C2 may each havecapacitance of approximately 7 nF. However, as one of ordinary skill inthe art will understand, the quantity of capacitance may be adjusted invarious embodiments depending on the application. In addition, thenumber and types of devices used to provide DC blocking capacitance mayvary in different embodiments.

The circuit 100 includes PMOS transistors T4 and T5, and NMOStransistors T6 and T7. As shown in FIG. 1, the transistors T4, T5, T6,and T7 are serially connected between VDD and VSS. The transistors T4,T5, T6, and T7 function as a multiplexer that selects between IN_VCC(post coupling capacitors C1, C2) as one input and IN_VDD as a secondinput. The control signals EN_VDD and its complement en_b_vdd serve toselect one of the two inputs. When EN_VDD is high and en_b_vdd is low,the first transmission gate T0/T1 is enabled, transistors T5 and T6 areoff, and the input IN_VCC is selected. When IN_VCC is the selectedinput, the AC component of the signal is present on the node N2 _(—)vdd. When EN_VDD is low and en_b_vdd is high, the first transmissiongate T0/T1 is disabled, transistors T5 and T6 are on, and IN_VDD is theselected input. When IN_VDD is the selected input, the IN_VDD signal maybe present on the gates of both transistor T4 and T7. Depending onwhether IN_VDD is high or low, IN_VDD will turn on one of the twotransistors T4, T7, while turning off the other transistor.

The circuit 100 includes a first inverter constructed from PMOStransistor T8 and NMOS transistor T9, which are serially arrangedbetween VDD and VSS. The input of first inverter T8/T9 is the node N2_(—) vdd. The output of first inverter T8/T9 is the node N3 _(—) vdd. Inaddition, the circuit 100 includes a feedback resistor R0 and a secondtransmission gate constructed from parallelly arranged PMOS transistorT10 and NMOS transistor T11. The second transmission gate T10/T11 has afirst terminal connected to input node N2 _(—) vdd and a second terminalconnected to feedback resistor R0 Like the first transmission gateT0/T1, the second transmission gate T10/T11 is enabled when EN_VDD ishigh and its complement en_b_vdd is low. As shown in FIG. 1, theresistor R0 and second transmission gate T10/T11 are arranged in series.As also shown in FIG. 1, the (a) first inverter T8/T9, and the (b)serially connected feedback resistor R0 and second transmission gateT10/T11 are arranged in parallel between the nodes N2 _(—) vdd and N3_(—) vdd. The feedback resistor R0 and second transmission gate T10/T11serve as a feedback path between the output and input of the firstinverter T8/T9.

As may be seen at the right side of FIG. 1, the circuit 100 includes asecond inverter constructed from PMOS transistor T12 and NMOS transistorT13, which are serially arranged between VDD and VSS. The input of thesecond inverter T12/T13 is node N3 _(—) vdd. The output of the secondinverter T12/T13 is output node OUT_VDD.

FIG. 1 also depicts a third inverter constructed from PMOS transistorT14 and NMOS transistor T15, which are serially arranged between VDD andVSS. The input of the third inverter T14/T15 is the EN_VDD controlsignal. The output of the third inverter T14/T15 is the complementen_b_vdd.

Operation of the circuit 100 is next described with reference to FIGS. 1and 2. In operation, when the control signal EN_VDD is high, the circuit100 is in a level-shifting mode. FIG. 2 depicts various waveforms for asimulation of the circuit of FIG. 1 when the circuit 100 is in thelevel-shifting mode. When EN_VDD is high, the first transmission gateT0/T1 is enabled, and T5 and T6 are off, so that the multiplexer selectsIN_VCC as the input. In addition, when EN_VDD is high, the secondtransmission gate T10/T11 is enabled, which opens the feedback pathbetween the output and input of the first inverter T8/T9. In thelevel-shifting mode, the AC component of the input signal IN_VCC will bepresent at node N2 _(—) vdd, which is also the input to the firstinverter T8/T9. The AC component of the input signal IN_VCC will beinverted by the first inverter T8/T9. Because the supply rails of thefirst inverter T8/T9 are VDD and VSS, the voltage level of the invertedcopy of the input signal IN_VCC will be level shifted from the VCCdomain. The level shifted copy present on node N3 _(—) vdd is input tothe second inverter T12/T13. The second inverter T12/T13 inverts thelevel-shifted signal, restoring it to its original logic value. DiagramA of FIG. 2 shows the input signal IN_VCC and the output signal OUT_VDD.It can be seen when the input signal IN_VCC is a logic one, the voltageis approximately 0.8V. And when the output signal OUT_VDD is a logicone, the voltage is approximately 1.1V.

Under normal operating conditions when the circuit 100 is inlevel-shifting mode, the IN_VCC input to the level shifter will beswitching regularly. The AC component of the input drives inverterT8/T9, which is powered from the VDD supply. The output of inverterT8/T9 is fed back to the input of the inverter through resistor R0. Thisfeedback effectively sets the DC level at node N2 _(—) vdd to VDD/2. Aslong as there is switching activity on the input, the AC component onnode N2 _(—) vdd will keep the input to the first inverter T8/T9 fromreaching a VDD/2 steady state, which would cause a high crowbar currentcoming from the first and second inverters.

Referring to diagram A of FIG. 2, it can be seen that when the inputsignal IN_VCC goes idle at about 12 ns, the output signal OUT_VDDsettles at about VDD/2 (about 0.5V) at about 17 ns. As long as is theinput IN_VCC is switching, the voltage on node N2 _(—) vdd oscillates,as shown in diagram C of FIG. 2. However, when the input IN_VCC is idle,the feedback path through resistor R0 will cause the DC level at node N2_(—) vdd to effectively settle at about VDD/2, as shown in diagram C ofFIG. 2. When the input is idle, both the input and output of firstinverter T8/T9 at nodes N2 _(—) vdd and N3 _(—) vdd, and the output ofthe second inverter T12/T13 at node N_OUT (output signal OUT_VDD), allsettle at about VDD/2 (about 0.5V), as can be seen in diagrams A and Cof FIG. 2. Diagram B of FIG. 2 depicts switching activity.

Referring to diagram D, a problem can be seen when the input stopsswitching. Diagram D of FIG. 2 shows current in the VDD power supply.When the input stops switching, there is no longer an AC component atnode N2 _(—) vdd. It can be seen that when the input is idle the node N2_(—) vdd is driven to a constant state of about VDD/2. With the gates oftransistors T8 and T9 at about VDD/2, neither transistor is off. Inother words, both transistors T8 and T9 are at least partially on,causing a current to flow between VDD and VSS. One of ordinary skill inart will recognize that a current of this type is referred to as a DC or“crowbar” current. Moreover, in addition to crowbar current in the firstinverter T8/T9, the voltage on N3 _(—) vdd at about VDD/2 will causecrowbar current in the second inverter T12/T13 for the same reason. Ascan be seen diagram D of FIG. 2, the power supply VDD draws around 700μA of current from about 15 ns until the input IN_VCC starts switchingagain at about 43 nS. In addition to the relatively large power draw,there may be an electro-migration problem.

FIGS. 3A and 3B depict an AC level shifter circuit 300 for levelshifting a time-varying input signal according to various embodiments.Devices in FIGS. 3A and 3B having the same reference numbers as devicesin FIG. 1 may refer to the same or a similar device. The AC levelshifter circuit 300 of FIGS. 3A and 3B is similar to the AC levelshifter circuit 100 of FIG. 1. However, some of the control signals thatthe circuit 300 uses differ from the circuit 100. In particular, firsttransmission gate T0/T1 of circuit 300 is controlled by control signalsen_n_vdd (coupled with the gate of T0) and en_p_vdd (coupled with thegate of T1). In addition, second transmission gate T10/T11 of circuit300 is controlled by control signals en_n_vdd (coupled with the gate ofT10) and en_p_vdd (coupled with the gate of T11).

Like the circuit 100, the circuit 300 includes PMOS transistors T4 andT5 and NMOS transistors T6 and T7. The transistors T4, T5, T6, and T7are serially connected between VDD and VSS, and function as amultiplexer that selects between IN_VCC (post coupling capacitors C1,C2) as one input and IN_VDD as a second input. However, control signalen_p_vdd (coupled with the gate of T5) and en_n_vdd (coupled with thegate of T6) are used to select one of the two inputs. Another differenceis that the circuit 300 does not include the third inverter T14/T15 ofthe circuit 100. Instead the third inverter T14/T15, the circuit 300includes sub-circuits 302 and 304 shown in FIG. 3B.

FIG. 3B shows a detection sub-circuit 302 for detecting an idle state oninput node N_IN and a control signal generation sub-circuit 304configured to modify a control signal in response to detection of anidle state on the input node. The detection sub-circuit 302 detects anidle state of the IN_VCC input signal and then breaks the feedback paththrough resistor R0 between the output and input of the first inverterT8/T9. According to the shown embodiment, the idle state of the IN_VCCinput signal is a constant logic low level. In alternative embodiments,the idle state of the IN_VCC input signal is a constant logic highlevel.

In the example of FIG. 3B, the detection sub-circuit 302 includescapacitors C3 and C4. The detection circuit also includes a PFETs T14,T15, T16, and T17, and an NFET T18. The PFETs T14, T15, T16, and T17 maybe referred to as a PFET pull-up stack. The sub-circuit 302 alsoincludes a resistor R1. One terminal of resistor R1 is defined as a“shutoff” node, while the other terminal is defined as a “shutoff_n”node.

The PFET pull-up stack T14, T15, T16, and T17 are serially coupledbetween VDD and the shutoff n node of resistor R1. The gates of thetransistors in the PFET pull-up stack are coupled with the IN_VCC inputsignal. The gate of NFET T18 is also connected to the IN_VCC inputsignal. The resistor R1 is connected at the shutoff_n node with asource/drain terminal of T17. The resistor R1 is connected at theshutoff node with capacitors C3 and C4, and a source/drain terminal ofNFET T18. The capacitors C3 and C4, and NFET T18 are connected inparallel between the shutoff node and VSS.

When IN_VCC is low, T18 is off, and the PFET pull-up stack and resistorR1 provide a path to pull up the voltage at the shutoff node. WhenIN_VCC is high, the transistors of the PFET pull-up stack are off, T18is on and it provides a path to pull down the voltage at the shutoffnode.

Still referring to FIG. 3B, the control signal generation sub-circuit304 includes an inverter 306 constructed from PMOS transistor T19 andNMOS transistor T20, which are serially arranged between VDD and VSS.The input of the inverter T19/T20 is the EN_VDD control signal. Theoutput of the inverter T19/T20 is provided to an inverter 308constructed from PMOS transistor T22 and NMOS transistor T23. Thetransistors T22 and T23, and a PMOS transistor T21 are serially arrangedbetween VDD and VSS. The gate of transistor T21, which enables ordisables the inverter 308, is coupled with the shutoff node. The outputof the inverter 308 is coupled with a source/drain terminal of an NMOStransistor T24 and an input of an inverter 310 constructed from PMOStransistor T25 and NMOS transistor T26. The other source/drain terminalof transistor T24 is coupled to VSS. The gate of transistor T24 iscoupled to the shutoff node. The transistor T24 pulls the input to theinverter 310 low when shutoff is high. The transistors T25 andtransistor T26 of inverter 310 are serially arranged between VDD andVSS. At the input of inverter 310 is a node that provides control signalen_p_vdd. At the output of inverter 310, T25/T26 is a node that providescontrol signal en_n_vdd.

The idle detection sub-circuit 302 includes one or more capacitors thatare regularly discharged in the presence of a switching IN_VCC inputsignal. In operation, the discharging of the capacitors C3 and C4 occurswhen the IN_VCC input signal is high. When the input signal is high, thetransistors of the PFET pull-up stack of the idle detection sub-circuit302 are off and NFET T18 is on. When T18 is on, the capacitors C3 and C4discharge via the path through NFET T18 to VSS. As the capacitorsdischarge, the shutoff node transitions low. When the IN_VCC inputsignal goes idle low, the transistors of the PFET pull-up stack are on,the NFET T18 is off, and the capacitors C3 and C4 charge up through theresistor R1 and the PFET pull up stack to VDD. As the capacitors chargeup, the shutoff node transitions high. As further described below, therate at which the capacitors C3, C4 charge is relatively slow comparedwith the rate at which they discharge in various embodiments.

When the shutoff node is low, transistor T21 of the control signalgeneration circuit 304 is on, and transistor T24 is off. When thetransistor T21 is on, inverter T22/T23 is enabled. When transistor T24is off, it does not influence node en_p_vdd. Accordingly, when theshutoff node is low, en_p_vdd is a copy of EN_VDD. If EN_VDD is high,en_p_vdd is high, and T5 and T6 are off, so that IN_VCC is selected asthe input to the level shifter 300. In addition, transmission gatesT0/T1 and T10/T11 are enabled.

As the shutoff node transitions high, transistor T21 of the controlsignal generation circuit 304 turns off, which disables the inverter308. In addition, as the shutoff node transitions high, the transistorT24 turns on. When the transistor T24 turns on, the node en_p_vdd ispulled low, and the output of inverter 310, en_n_vdd, goes high.Accordingly, when the shutoff node transitions high, en_p_vdd is low.When en_p_vdd is low, T5 and T6 are on, so that IN_VCC is selected asthe input to the level shifter 300. In addition, transmission gatesT0/T1 and T10/T11 are disabled.

When en_p_vdd is low, en_n_vdd is high, and the transmission gateT10/T11 disabled, the feedback path for the first inverter T8/T9 isbroken. With the feedback path broken, node N2 _(—) vdd either goes highor low, depending on whether IN_VDD is high or low. In the examplewaveforms of FIG. 4, IN_VDD is high, so node N3 _(—) vdd will transitionlow at a first time after the input IN_VCC stops switching. With thefeedback path broken, the crowbar current in inverter T8/T9 falls towardzero. In an alternative example, IN_VDD may be low.

When the input signal IN_VCC resumes switching, the capacitors C3 and C4will once again be discharged (the shutoff node will be driven low), andtherefore the feedback passgate T10/T11 will again be transparent. Uponthis re-enabling the of feedback path, the level shifter 300 will resumeoperation in level-shifting mode, with IN_VCC as the selected input.

In various embodiments, the rate at which the capacitors C3, C4 chargeis relatively slow compared with the rate at which they discharge. Thedifference in the charge and discharge rates is due to the difference ineffective widths of the transistors in the charge and discharge paths.Specifically, the transistors of the PFET pull-up stack are relativelysmall as compared to the relatively large NFET T18 in pull down path. Inaddition, the resistance provided by R1 increases the time constant forcharging up the capacitors C3 and C4. The difference in the charge anddischarge rates allows the level shifter 300 to remain in level-shiftingmode for a first time period. If the input is idle for a time periodless than the first time period, the level shifter remains inlevel-shifting mode. It is only when the input is idle for a time periodthat extends beyond the first period that the level shifter exits thelevel shifting mode by opening the feedback path. The first time periodcan be configured for a particular application by selecting appropriatecomponent values and sizes.

FIG. 4 shows waveforms for a simulation of the level shifter circuit300, which includes the idle detection sub-circuit 302 and the controlsignal generation sub-circuit 304. Diagram A of FIG. 4 depicts the inputsignal IN_VCC and the output signal OUT_VDD. Diagram D of FIG. 4 showswaveforms for shutoff and shutoff_n nodes. It can be seen that the inputsignal IN_VCC is initially switching, but goes idle at about 12 ns.During the period when IN_VCC is switching, it can be seen in diagram Dthat the shutoff node slowly rises well under 0.25V and falls back tozero. In diagram A, it can be seen that the input signal IN_VCC goesidle at about 12 ns. When the input goes idle (and IN_VCC is low),diagram D shows that the voltage on the shutoff node rises as capacitorsC3 and C4 charge up.

After the input goes idle at about 12 ns, the output signal OUT_VDDinitially settles at about VDD/2. At around 20 ns, the shutoff nodereaches 0.5V on its transition to a high state. As can be seen indiagram B, the control signal en_n_vdd transitions high and the controlsignal en_p_vdd transitions low at about 23 ns. It can be seen indiagram A that the output signal OUT_VDD rises to 1.1V at about thistime. In addition, as shown in diagram E, current in VDD which rose to750 μA after the input went idle falls toward zero.

Diagram C of FIG. 4 shows waveforms for node N2 _(—) vdd and node N3_(—) vdd. As long as is the input IN_VCC is switching, the voltages onnode N2 _(—) vdd and node N3 _(—) vdd oscillate. When the input IN_VCCis idle, these two nodes settle at about VDD/2. Referring to diagram E,the power supply VDD draws on around 750 μA of current while these nodesare at VDD/2. However, when the control signal en_n_vdd transitions highand the control signal en_p_vdd transitions low at about 23 ns, node N2_(—) vdd goes high, node N3 _(—) vdd goes low, and current in the powersupply VDD drops toward zero.

The current from the power supply VDD for the examples presented abovemay be compared. In the waveforms of FIG. 2, the power supply VDD drawsaround 700 μA of current for about 28 ns. In the waveforms of FIG. 4,the power supply VDD draws around 750 μA of current for about 11 ns. Theinventors have performed simulations that show that the average currenton VDD in level shifter circuit 100 is 377 uA, whereas the level shiftercircuit 300 draws an average current on VDD of 244 uA. This is a 35%reduction in current for a level shifter circuit that is idle about 50%of the time, as shown in the waveforms in FIG. 4. With longer idle timesthe power savings would increase dramatically.

FIG. 5 shows a block diagram of an exemplary design flow 500 used forexample, in semiconductor IC logic design, simulation, test, layout, andmanufacture. Design flow 500 includes processes, machines and/ormechanisms for processing design structures or devices to generatelogically or otherwise functionally equivalent representations of thedesign structures and/or devices described above and shown in FIGS. 1-4.The design structures processed and/or generated by design flow 500 maybe encoded on machine-readable transmission or storage media to includedata and/or instructions that when executed or otherwise processed on adata processing system generate a logically, structurally, mechanically,or otherwise functionally equivalent representation of hardwarecomponents, circuits, devices, or systems. Machines include, but are notlimited to, any machine used in an IC design process, such as designing,manufacturing, or simulating a circuit, component, device, or system.For example, machines may include: lithography machines, machines and/orequipment for generating masks (e.g. e-beam writers), computers orequipment for simulating design structures, any apparatus used in themanufacturing or test process, or any machines for programmingfunctionally equivalent representations of the design structures intoany medium (e.g. a machine for programming a programmable gate array).

Design flow 500 may vary depending on the type of representation beingdesigned. For example, a design flow 500 for building an applicationspecific IC (ASIC) may differ from a design flow 500 for designing astandard component or from a design flow 500 for instantiating thedesign into a programmable array, for example a programmable gate array(PGA) or a field programmable gate array (FPGA) offered by Altera® Inc.or Xilinx® Inc.

FIG. 5 illustrates multiple such design structures including an inputdesign structure 520 that is preferably processed by a design process510. Design structure 520 may be a logical simulation design structuregenerated and processed by design process 510 to produce a logicallyequivalent functional representation of a hardware device. Designstructure 520 may also or alternatively comprise data and/or programinstructions that when processed by design process 510, generate afunctional representation of the physical structure of a hardwaredevice. Whether representing functional and/or structural designfeatures, design structure 520 may be generated using electroniccomputer-aided design (ECAD) such as implemented by a coredeveloper/designer. When encoded on a machine-readable datatransmission, gate array, or storage medium, design structure 520 may beaccessed and processed by one or more hardware and/or software moduleswithin design process 510 to simulate or otherwise functionallyrepresent an electronic component, circuit, electronic or logic module,apparatus, device, or system such as those shown in FIGS. 1, 3A, and 3B.As such, design structure 520 may comprise files or other datastructures including human and/or machine-readable source code, compiledstructures, and computer-executable code structures that when processedby a design or simulation data processing system, functionally simulateor otherwise represent circuits or other levels of hardware logicdesign. Such data structures may include hardware-description language(HDL) design entities or other data structures conforming to and/orcompatible with lower-level HDL design languages such as Verilog andVHDL, and/or higher level design languages such as C or C++.

Design process 510 preferably employs and incorporates hardware and/orsoftware modules for synthesizing, translating, or otherwise processinga design/simulation functional equivalent of the components, circuits,devices, or logic structures shown in FIGS. 1, 3A, and 3B to generate aNetlist 580 which may contain design structures such as design structure520. Netlist 580 may comprise, for example, compiled or otherwiseprocessed data structures representing a list of wires, discretecomponents, logic gates, control circuits, I/O devices, models, etc.that describes the connections to other elements and circuits in anintegrated circuit design. Netlist 580 may be synthesized using aniterative process in which netlist 580 is resynthesized one or moretimes depending on design specifications and parameters for the device.As with other design structure types described herein, netlist 580 maybe recorded on a machine-readable data storage medium or programmed intoa programmable gate array. The medium may be a non-volatile storagemedium such as a magnetic or optical disk drive, a programmable gatearray, a compact flash, or other flash memory. Additionally, or in thealternative, the medium may be a system or cache memory, buffer space,or electrically or optically conductive devices and materials on whichdata packets may be transmitted and intermediately stored via theInternet, or other networking suitable means.

Design process 510 may include hardware and software modules forprocessing a variety of input data structure types including Netlist580. Such data structure types may reside, for example, within libraryelements 530 and include a set of commonly used elements, circuits, anddevices, including models, layouts, and symbolic representations, for agiven manufacturing technology (e.g., different technology nodes, 32 nm,45 nm, 90 nm, etc.). The data structure types may further include designspecifications 540, characterization data 550, verification data 560,design rules 570, and test data files 585 which may include input testpatterns, output test results, and other testing information. Designprocess 510 may further include, for example, standard mechanical designprocesses such as stress analysis, thermal analysis, mechanical eventsimulation, process simulation for operations such as casting, molding,and die press forming, etc. One of ordinary skill in the art ofmechanical design can appreciate the extent of possible mechanicaldesign tools and applications used in design process 510 withoutdeviating from the scope and spirit of the invention. Design process 510may also include modules for performing standard circuit designprocesses such as timing analysis, verification, design rule checking,place and route operations, etc.

Design process 510 employs and incorporates logic and physical designtools such as HDL compilers and simulation model build tools to processdesign structure 520 together with some or all of the depictedsupporting data structures along with any additional mechanical designor data (if applicable), to generate a second design structure 590.Design structure 590 resides on a storage medium or programmable gatearray in a data format used for the exchange of data of mechanicaldevices and structures (e.g. information stored in a IGES, DXF,Parasolid XT, JT, DRG, or any other suitable format for storing orrendering such mechanical design structures). Similar to designstructure 520, design structure 590 preferably comprises one or morefiles, data structures, or other computer-encoded data or instructionsthat reside on transmission or data storage media and that whenprocessed by an ECAD system generate a logically or otherwisefunctionally equivalent form of one or more of the embodiments of theinvention shown in FIGS. 1-4. In one embodiment, design structure 590may comprise a compiled, executable HDL simulation model thatfunctionally simulates the devices shown in FIGS. 1, 3A, and 3B.

Design structure 590 may also employ a data format used for the exchangeof layout data of integrated circuits and/or symbolic data format (e.g.information stored in a GDSII (GDS2), GL1, OASIS, map files, or anyother suitable format for storing such design data structures). Designstructure 590 may comprise information such as, for example, symbolicdata, map files, test data files, design content files, manufacturingdata, layout parameters, wires, levels of metal, vias, shapes, data forrouting through the manufacturing line, and any other data required by amanufacturer or other designer/developer to produce a device orstructure as described above and shown in FIGS. 1-4. Design structure590 may then proceed to a stage 595 where, for example, design structure590: proceeds to tape-out, is released to manufacturing, is released toa mask house, is sent to another design house, is sent back to thecustomer, etc.

The descriptions of the various embodiments of the present disclosurehave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

1. A level shifter circuit for coupling a first circuit with a secondcircuit, the first circuit using a first supply voltage and the secondcircuit using a second supply voltage, comprising: an input node toreceive an input signal; an output node to output to a level-shiftedoutput signal corresponding with the input signal; a detectionsub-circuit to detect an idle state on the input node, the idle statebeing a state that corresponds with a particular binary logic value andwhich is maintained for a first time period; a first inverter that usesthe second supply voltage, the first inverter having an input and anoutput; and a feedback path between the input and output of the firstinverter, the feedback path including a first resistive element and afirst transmission gate, the first transmission gate configurable toopen the feedback path when the detection sub-circuit detects an idlestate on the input node of the level shifter circuit.
 2. The levelshifter circuit of claim 1, wherein the detection sub-circuit comprisesa capacitive element and a second resistive element, and the first timeperiod is determined by the capacitive element and the second resistiveelement.
 3. The level shifter circuit of claim 1, wherein the detectionsub-circuit comprises: a second resistive element having a first and asecond node; a capacitive element, the capacitive element beingconnected between the first node and a negative rail of a supplyvoltage; a first switching device serially connected between the secondnode and a positive rail of the supply voltage; a second switchingdevice connected between the first node and the negative rail of thesupply voltage, and in parallel with the capacitive element; whereinwhen the first switching device is closed, the capacitive element, thesecond resistive element, and the first switching device are seriallyconnected between positive and negative rails of a supply voltage toprovide a path to charge the capacitive element; and wherein when thesecond switching device is closed, the second switching device isconnected between the first node and the negative rail of the supplyvoltage, providing a path to discharge the capacitive element.
 4. Thelevel shifter circuit of claim 3, wherein the first and second switchingdevices are operated by the input signal on the input node, and whereinwhen the first switching device is open, the second switching device isclosed.
 5. The level shifter circuit of claim 3, wherein the voltage atthe first node provides an indication as to whether the input node is inthe idle state.
 6. The level shifter circuit of claim 1, wherein theinput of the first inverter is coupled with a multiplexer, themultiplexer being configurable to select a first or a second inputsignal.
 7. The level shifter circuit of claim 6, wherein the first inputsignal is input to the input node of the level shifter circuit and themultiplexer is configured to select the second input signal when thedetection sub-circuit detects an idle state on the input node.
 8. Thelevel shifter circuit of claim 1, further comprising a control signalgeneration sub-circuit to generate a control signal in response todetection of the idle state on the input node.
 9. The level shiftercircuit of claim 1, wherein the particular binary logic value is a logiczero.
 10. The level shifter circuit of claim 1, further comprising acapacitive element coupled between the input node and the input of thefirst inverter, the capacitive element configured to pass an ACcomponent of the input signal.
 11. (canceled)
 12. (canceled)
 13. Adesign structure tangibly embodied in a machine readable storage mediumused in a design process, the design structure specifying a levelshifter circuit for coupling a first circuit with a second circuit, thefirst circuit using a first supply voltage and the second circuit usinga second supply voltage, the design structure comprising: an input nodeto receive an input signal; an output node to output to a level-shiftedoutput signal corresponding with the input signal; a detectionsub-circuit to detect an idle state on the input node, the idle statebeing a state that corresponds with a particular binary logic value andwhich is maintained for a first time period; a first inverter that usesthe second supply voltage, the first inverter having an input and anoutput; and a feedback path between the input and output of the firstinverter, the feedback path including a first resistive element and afirst transmission gate, the first transmission gate configurable toopen the feedback path when the detection sub-circuit detects an idlestate on the input node of the level shifter circuit.
 14. The designstructure of claim 13, wherein the design structure comprises a netlist,which describes the SRAM local evaluation circuit.
 15. The designstructure of claim 13, wherein the design structure resides on storagemedium as a data format used for the exchange of layout data ofintegrated circuits.
 16. The design structure of claim 13, wherein thedesign structure includes at least one of test data files,characterization data, verification data, or design specifications. 17.The design structure of claim 13, wherein the detection sub-circuitcomprises: a second resistive element having a first and a second node;a capacitive element, the capacitive element being connected between thefirst node and a negative rail of a supply voltage; a first switchingdevice serially connected between the second node and a positive rail ofthe supply voltage; a second switching device connected between thefirst node and the negative rail of the supply voltage, and in parallelwith the capacitive element; wherein when the first switching device isclosed, the capacitive element, the second resistive element, and afirst switching device are serially connected between positive andnegative rails of a supply voltage to provide a path to charge thecapacitive element; and wherein when the second switching device isclosed, the second switching device is connected between the first nodeand the negative rail of the supply voltage, providing a path todischarge the capacitive element.
 18. The design structure of claim 17,wherein the first and second switching devices are operated by the inputsignal on the input node, and wherein when the first switching device isopen and second switching device is closed, and wherein the voltage atthe first node provides an indication as to whether the input node is inan the idle state.
 19. The design structure of claim 13, wherein theinput of the first inverter is coupled with a multiplexer, themultiplexer being configurable to select a first or a second inputsignal, and wherein the first input signal is input to the input node ofthe level shifter circuit and the multiplexer is configured to selectthe second input signal when the detection sub-circuit detects an idlestate on the input node.
 20. The design structure of claim 13, furthercomprising a control signal generation sub-circuit to generate a controlsignal in response to detection of the idle state on the input node.